A field effect transistor is a device generally having a semiconductor layer (a semiconductor film), a source electrode, a drain electrode, a gate electrode opposed to these electrodes via an insulating layer, and others on a substrate. The field effect transistor has been widely used not only as a logical circuit element in integrated circuits but also as a switching element or the like. A semiconductor layer is usually made of a semiconductor material. At present, inorganic semiconductor materials mainly including silicon have been used in field effect transistors. Particularly, a thin-film transistor having a semiconductor layer formed from amorphous silicon on a substrate of glass or the like has been used in displays and others. In the case of using such inorganic semiconductor materials, treatments at high temperature or in vacuum are required during manufacturing of field effect transistors. Accordingly, expensive equipment investment and high energy consumption are required for manufacturing, resulting in extremely high costs. In addition, since these materials are exposed to high temperature during manufacturing of field effect transistors, materials having insufficient heat resistance, e.g., a film or a plastic, cannot be used as a substrate. Consequently, a flexible material having bendability or the like cannot be used as a substrate, resulting in the limited applications thereof.
Meanwhile, research and development of field effect transistors with an organic semiconductor material have been actively performed. Use of an organic material enables manufacture by a low-temperature process requiring no treatment at high temperature, extending the range of materials that can be used for a substrate. As a result, it is feasible to manufacture more flexible, lighter and more irrefrangible field effect transistors than were possible. Furthermore, in a manufacturing process of field effect transistors, an application method using a solution of a semiconductor material and a printing method with inkjet or the like can be employed in some instances. Accordingly, large-area field effect transistors may be manufactured at a low cost. Moreover, because various types of compounds can be selected for an organic semiconductor material, it has been expected to develop novel functions based on the characteristics thereof.
Various studies have been conducted so far on use of an organic compound as a semiconductor material. For example, semiconductor materials from a pentacene, a thiophene, or an oligomer or a polymer thereof have been already known as ones having hole transport characteristics (see Patent Documents 1 and 2). Pentacene is an acene-type aromatic hydrocarbon having 5 linearly-fused benzene rings. A field effect transistor using a pentacene as a semiconductor material has been reported to have equivalent charge mobility (carrier mobility) to that of amorphous silicon in practical use. However, field effect transistors with a pentacene are environmentally-degradable and questioned in stability. The same problems are also caused when a thiophene compound is used. Accordingly, neither of the compounds would be highly useful in practice. Under the circumstances, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) with high stability in the air was developed and is receiving attention (see Patent Document 3 and Non Patent Document 1).
However, even higher carrier mobility is required for use of these compounds in display application such as organic EL, and improvement in solubility is also required for producing a field effect transistor by an application method such as printing, which also reflects strong market demands. Furthermore, from the viewpoint of durability, development of an organic semiconductor material having high quality and high performance is required. According to prior art on DNTT derivatives having a substituent group such as Patent Documents 3, 4, and 5, examples of the specific substituent groups include a methyl group, a hexyl group, an alkoxyl group, and a substituted ethynyl group, and only the methyl group and the substituted ethynyl group are disclosed as a substituent group of the DNTT derivatives in examples, each only exhibiting semiconductor characteristics that are similar to or poorer than those of the DNTT having no substituent group under the present circumstances.